Post-image plugin thumbnails don’t link to the original image

Hello, I have a little problem on my site http://www.karakter.pl. I needed an image to be displayed next to a title’s post, so I installed a Post-Image plugin, then I put this code into my template:

<?php post_image('','use_thumb=1') ;?>

It works perfectly, except for the fact that although the image in full resolution uploads correctly to the Uploads folder and also the thumbnail is generated all right, I cannot make the thumbnail link to the original image. My total lack of knowledge of PHP doesn’t help, so I would be very grateful if someone found a way to fix this.

Cheers,

k